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Linkwitz Transform

Discussion in 'Home Theater Projects' started by RickVS, Jun 24, 2005.

  1. RickVS

    RickVS Member

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    I have a sub that I can only put in a box that is a bit too small. I was going to do a feedback system, but came across the info for a Linkwitz Transform. Seems pretty much dead on with what I want to accomplish.

    Working with the value a bit, it seems extremely touchy! Just a small shift in box volume throws everything off. Has anyone had experience setting these circuits up? Currently it seems like the best way to do it is going to be to build the speaker box, and test the woofers actual in box frequency response. Take the results and determine the Q and Fb to build the Linkwitz transform. Is this really the best way?
     
  2. Dan Wesnor

    Dan Wesnor Well-Known Member

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    Put the woofer in the box just like it will be in the final - including the amp, if applicable - and measure Fs and Qts directly using something like SpeakerWorkshop (free). You'll have to bypass the amp for the measurements, but you want it taking up space. Make sure you're using and amp with no boost in the final design.
     
  3. Allen Ross

    Allen Ross Well-Known Member

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    i would agree, but also look at the extension of the driver and power on the amp to be sure that you can use a LT.
     
  4. RickVS

    RickVS Member

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    Dan, by Fs and Qts, do you mean Fb and Qtc? Those were what I thought I was supposed to be using. Have you used Linkwitz transforms? Are they as touchy as they seem on paper? I can't imagine the real world being easier then paper
     
  5. JimPeitersen

    JimPeitersen Well-Known Member

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    Rick,
    I sent you a PM about LT's.
    JP
     
  6. Dan Wesnor

    Dan Wesnor Well-Known Member

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    If the woofer is in the box, Fb=Fs and Qts=Qtc.
     
  7. RickVS

    RickVS Member

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    Dan - I thought in a sealed enclosure:
    Fs = Free air resonance of the woofer
    Fb = System tuning frequency
     
  8. ThomasW

    ThomasW Well-Known Member

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  9. Geoff L

    Geoff L Well-Known Member

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    Unless you have already got an amp and are planning to build your own LT, you might consider "talking with Brian" over at www.RythmikAudio.com about a possible plate amp with an LT built in.

    Rythmik's LT page is *HERE* Even if not interested, there is some nice reading, graphs, and a couple links their also I belive..

    It would, has to be designed to work with your box and your driver, assuming your driver is applicable for the project. Xmax & power handling being a concern if their on the lower side of things.

    He dose them on the 250-watt and 380-watt Basic Plate Amps. The LT can be taken out of the equation with a flip of a switch. In other words turned on and off by a toggle. I have 2 of the Basic-350's and love them, NON LT'ed tho...! They do 380-watts/4-ohm even tho called the 350.

    Very reasonable in price also, I belive 40.00 LT & toggle installed and complete, or 15.00 with LT-board in and toggle mounted, but you have to solder the sent HQ components up.

    Just another place to look, (if your lazy or have less than great soldering tools & skills), like me, :b & can get it all done in one shot.

    Cheers ¥
     
  10. ThomasW

    ThomasW Well-Known Member

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  11. RickVS

    RickVS Member

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    I like that one Thomas. I like the inclusion of the subsonic filter in the design, as well as the woofer to see where you are hitting Xmax and power limits. Would be nice if the speaker information was as robust as UniBox
     
  12. Dan Wesnor

    Dan Wesnor Well-Known Member

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    Fb = F box. Technically, Fb is the resonant frequency of the box by itself. If you look at ported boxes, you will see that Fb has nothing to do with the woofer. The same is true in sealed boxes, although the distinction is typically ignored.

    Fs = F system, which is ambiguous. If there is not a box, system refers to the combination of the mechanical and electrical filters in the driver, so Fs includes Fms and Fes. If there is a box in the system Fs includes the box (Fb) as well as Fms and Fes.

    We don't make a distinction between Fms and Fes (like we do for Qms and Qes) because it doesn't add anything for our purposes (Qes is used to calculate efficiency, so separating Qms and Qes is useful). So Fs is the combination of Fb, Fms, and Fes. Fms is in turn a combination of the various mechanical bits, the spider, the surround, and the air resistance on the cone.

    Either way, I don't think anybody was confused when I said to stick it in the box and measure it. [​IMG]
     
  13. KurtJ

    KurtJ Well-Known Member

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    Thought I'd post my questions here since it's about LT and I didn't want to start another thread.

    I'm currently running Atlas 15 sealed, 3.7ft^3, Qts .37, Qtc .707, 250W Rythmik Amp. WAF has gone way down since I built it last January (I was the guy who blew up one).

    Been modeling LT (Qts .37, Qtc .577, Fs 21.5Hz)in winISD and noticed that by going to 350W 2ft^3, with LT my output is down about 3dB from what it is now. Does that sound about right? Just want to make sure my calcs are pretty close before calling Rythmik.
     

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